1. Field of the Invention
The present invention relates to a rubber modified styrene based resin composition which is improved in a balance between impact strength and appearance of shaped articles such as gloss and image clarity.
2. Description of the Related Art
In order to improve the impact strength of hard brittle styrene based resins, it is well known that a rubbery elastomer is blended with a styrene based polymer, or that a rubbery elastomer is dissolved in a styrene monomer, followed by polymerization to prepare a modified styrene based resin composition. However, the styrene based resins are disadvantageous in that they are poor in a balance between impact strength and appearance of shaped articles compared to the ABS resins which are widely used as housing materials for household electric apparatuses and office systems.
A method for improving the appearance (gloss and inage clarity) of the shaped articles by reducing the particle size of the rubber has been heretofore investigated. However, the reduction in particle size to an extent as achieving a satisfactory gloss leads to a great reduction in impact strength, Thus, no resin has been obtained which is good in the balance.
For the purpose of improvement in the balance between impact strength and gloss, a composition comprising a dispersion of a smaller size particulate rubber component and a larger size particulate rubber component is disclosed in prior art publications such as U.S. Pat. No. 4,493,922, Japanese Patent Kokoku (Post-Exam. Publication) No. Hei 1-41177, Japanese Patent KOKAI (Laid-open) No. Hei 1-261444, and Japanese Patent KOKAI (Laid-open) No. Sho 63-112646. If the larger particle size rubber component has a particle size of 2 .mu.m or more, the appearance of the flow ends of the injected molded articles such as gloss and especially image clarity may degrade, while if it has a particle size of less than 2 .mu.m, insufficient impact strength is attained. Either case can not achieve a composition having a combination of fully satisfactory impact strength and appearance of the shaped articles.
A composition comprising a rubber modified styrene based resin, to which an organic polysiloxane is added for improving the impact strength is disclosed in prior art publications such as Japanese Patent Kokoku (Post-Exam. Publication) No. Sho 63-26774 and Japanese Patent Kokoku (Post-Exam. Publication) No. Sho 63-51459, though a composition comprising with smaller size rubber particles of less than 0.5 .mu.m could not achieve satisfactory impact strength.
As used here, the term "image clarity" is a measure of goodness of the appearance of the shaped articles. In addition of the image clarity, a degree of gloss is also known as a measure of goodness of the appearance of the shaped articles, though the image clarity allows for a higher level of evaluation in quality than the gloss.